1. Field of the Invention
The invention relates to an occupant detection system for detecting if an occupant is present in a vehicle seat, and more particularly to an occupant detection system that is able to detect an occupant with stability even when the vehicle seat is wet, for example.
2. Description of the Related Art
In automobiles, detected information as to whether an occupant is present in a seat is used for determining whether an air bag is to be deployed or inflated. A vehicle air-bag system is controlled so that an air bag deploys when an occupant is present in a seat, such as a passenger's seat, at the time of a collision of the vehicle, and the air bag does not deploy when no occupant is present in the seat. Also, in the case where the occupant sitting in the seat is a child having a small body size, for example, the air bag is required not to deploy even at the time of a collision of the vehicle. While various methods for detecting a seating condition of an occupant or the body size or shape of the occupant have been used, a capacitance-based occupant detection system is known, for example. Since a human body is a dielectric, the capacitance introduced between a detection electrode provided in a seat bottom or seatback portion of the seat and the vehicle body varies depending upon whether an occupant is present in the seat or no occupant is present in the seat. If an electric field is generated around the detection electrode, disturbances arise in the electric field when an occupant is seated in the seat. The capacitance-based occupant detection system detects the variation in the capacitance by detecting a change of voltage or current or disturbances in the electric field, for example, so as to determine whether an occupant is seated. An occupant detection system including a plurality of strip antenna electrodes disposed in horizontal positions while being vertically spaced from each other, in a seatback portion of a seat, is disclosed (see Japanese Patent Application Publication No. 2000-75044 (JP-A-2000-75044)), as an occupant detection system that accurately detects if the occupant is an adult or a child, and permits an air-bag device to be deployed when an adult is seated in the seat.
In the capacitance-based occupant detection system, the detection electrode or electrodes used for detecting an occupant is/are disposed on the surface of the vehicle seat or immediately below the seat surface. Therefore, if the seat gets wet, the impedance around the detection electrode(s) may change, or a wet portion may act as an antenna, which may result in a problem that a determination as to the presence of a seated occupant or whether the occupant is an adult or a child is erroneously made. In order to prevent or reduce the possibility of erroneous detection of an occupant due to a wet condition of the seat, an occupant detection system (as disclosed in Japanese Patent Application Publication No. 2007-240515 (JP-A-2007-240515)) has been proposed which applies a load current to an antenna electrode provided in the seat so as to generate a very weak electric field, while detecting a potential current that passes through the antenna electrode, and calculates the impedance and phase difference from the load current and the potential current, so as to detect an occupant based on the calculated impedance and phase difference.
The occupant detection system disclosed in JP-A-2000-75044 causes a very weak electric field to be selectively generated around the above-indicated plurality of antenna electrodes, and extracts or finds the shoulder line of the occupant by detecting current that passes through the antenna electrodes, so as to detect if the occupant is an adult or a child. However, in a condition where the back and head of the occupant are not in contact with the seatback of the seat, the system may not be able to accurately discriminate between an adult and a child. For example, if the occupant sits in a forward leaning position, it is impossible to discriminate between an adult and a child. Also, a large number of electrodes need to be installed in the seatback portion of the seat, and a switching circuit, or the like, for selectively using the electrodes needs to be provided, thus making the configuration and control of the system complicated.
In the occupant detection system disclosed in JP-A-2007-240515, the impedance and phase difference are calculated from the load current and potential current of the antenna electrode provided in the seat, and an occupant is detected based on the calculated impedance and phase difference, for prevention of erroneous detection due to wetting of the seat, for example. However, there is a need to provide electrodes for proximity measurement, which are used for measuring the impedance and the phase difference. Also, a complicated operation or processing needs to be performed to calculate the impedance and the phase difference from the measured load current and potential current, determine a threshold value based on the calculated phase difference, and compares the impedance with the threshold value. As described above, it is difficult or impossible for the known occupant detection systems to accurately determine the body size of a seated occupant (e.g., discriminate between an adult and a child), and also make the determination with accuracy even when there is a disturbance, such as wetting of the seat. Also, it is necessary to provide a plurality of electrodes for solving the respective problems as described above, which makes the configuration and processing complicated.